XML

1
XML

• Semistructured Data
• Extensible Markup Language
• Document Type Definitions

2
Semistructured Data

• Another data model, based on trees.
• Motivation flexible representation of data.
• Often, data comes from multiple sources with
  differences in notation, meaning, etc.
• Motivation sharing of documents among systems
  and databases.

3
Graphs of Semistructured Data

• Nodes objects.
• Labels on arcs (attributes, relationships).
• Atomic values at leaf nodes (nodes with no arcs
  out).
• Flexibility no restriction on
• Labels out of a node.
• Number of successors with a given label.

4
Example Data Graph
root
beer
beer
bar
manf
manf
prize
A.B.
name
name
year
award
servedAt
Bud
Gold
1995
Mlob
name
addr
Maple
Joes
5
XML

• XML Extensible Markup Language.
• While HTML uses tags for formatting (e.g.,
  italic), XML uses tags for semantics (e.g.,
  this is an address).
• Key idea create tag sets for a domain (e.g.,
  genomics), and translate all data into properly
  tagged XML documents.

6
Well-Formed and Valid XML

• Well-Formed XML allows you to invent your own
  tags.
• Similar to labels in semistructured data.
• Valid XML involves a DTD (Document Type
  Definition), a grammar for tags.

7
Well-Formed XML

• Start the document with a declaration, surrounded
  by lt?xml ?gt .
• Normal declaration is
• lt?xml version 1.0 standalone yes ?gt
• Standalone no DTD provided.
• Balance of document is a root tag surrounding
  nested tags.

8
Tags

• Tags, as in HTML, are normally matched pairs, as
  ltFOOgt lt/FOOgt .
• Tags may be nested arbitrarily.
• XML tags are case sensitive.

9
Example Well-Formed XML

• lt?xml version 1.0 standalone yes ?gt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt
• ltPRICEgt2.50lt/PRICEgtlt/BEERgt
• ltBEERgtltNAMEgtMillerlt/NAMEgt
• ltPRICEgt3.00lt/PRICEgtlt/BEERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

10
XML and Semistructured Data

• Well-Formed XML with nested tags is exactly the
  same idea as trees of semistructured data.
• We shall see that XML also enables nontree
  structures, as does the semistructured data model.

11
Example

• The ltBARSgt XML document is

BARS
BAR
BAR
BAR
NAME
. . .
BEER
BEER
Joes Bar
PRICE
PRICE
NAME
NAME
Bud
2.50
Miller
3.00
12
DTD Structure

• lt!DOCTYPE ltroot taggt
• lt!ELEMENT ltnamegt(ltcomponentsgt)gt
• . . . more elements . . .
• gt

13
DTD Elements

• The description of an element consists of its
  name (tag), and a parenthesized description of
  any nested tags.
• Includes order of subtags and their multiplicity.
• Leaves (text elements) have PCDATA (Parsed
  Character DATA ) in place of nested tags.

14
Example DTD

• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR)gt
• lt!ELEMENT BAR (NAME, BEER)gt
• lt!ELEMENT NAME (PCDATA)gt
• lt!ELEMENT BEER (NAME, PRICE)gt
• lt!ELEMENT PRICE (PCDATA)gt
• gt

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Element Descriptions

• Subtags must appear in order shown.
• A tag may be followed by a symbol to indicate its
  multiplicity.
• zero or more.
• one or more.
• ? zero or one.
• Symbol can connect alternative sequences of
  tags.

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Example Element Description

• A name is an optional title (e.g., Prof.), a
  first name, and a last name, in that order, or it
  is an IP address
• lt!ELEMENT NAME (
• (TITLE?, FIRST, LAST) IPADDR
• )gt

17
Use of DTDs

• Set standalone no.
• Either
• Include the DTD as a preamble of the XML
  document, or
• Follow DOCTYPE and the ltroot taggt by SYSTEM and a
  path to the file where the DTD can be found.

18
Example (a)

• lt?xml version 1.0 standalone no ?gt
• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR)gt
• lt!ELEMENT BAR (NAME, BEER)gt
• lt!ELEMENT NAME (PCDATA)gt
• lt!ELEMENT BEER (NAME, PRICE)gt
• lt!ELEMENT PRICE (PCDATA)gt
• gt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt ltPRICEgt2.50lt/PRICEgtlt/BEER
  gt
• ltBEERgtltNAMEgtMillerlt/NAMEgt ltPRICEgt3.00lt/PRICEgtlt/B
  EERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

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Example (b)

• Assume the BARS DTD is in file bar.dtd.
• lt?xml version 1.0 standalone no ?gt
• lt!DOCTYPE BARS SYSTEM bar.dtdgt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt
• ltPRICEgt2.50lt/PRICEgtlt/BEERgt
• ltBEERgtltNAMEgtMillerlt/NAMEgt
• ltPRICEgt3.00lt/PRICEgtlt/BEERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

20
Attributes

• Opening tags in XML can have attributes.
• In a DTD,
• lt!ATTLIST E . . . gt
• declares an attribute for element E, along with
  its datatype.

21
Example Attributes

• Bars can have an attribute kind, a character
  string describing the bar.
• lt!ELEMENT BAR (NAME BEER)gt
• lt!ATTLIST BAR kind CDATA IMPLIEDgt

22
Example Attribute Use

• In a document that allows BAR tags, we might see
• ltBAR kind sushigt
• ltNAMEgtAkasakalt/NAMEgt
• ltBEERgtltNAMEgtSapporolt/NAMEgt
• ltPRICEgt5.00lt/PRICEgtlt/BEERgt
• ...
• lt/BARgt

23
IDs and IDREFs

• Attributes can be pointers from one object to
  another.
• Compare to HTMLs NAME foo and HREF foo.
• Allows the structure of an XML document to be a
  general graph, rather than just a tree.

24
Creating IDs

• Give an element E an attribute A of type ID.
• When using tag ltE gt in an XML document, give its
  attribute A a unique value.
• Example
• ltE A xyzgt

25
Creating IDREFs

• To allow objects of type F to refer to another
  object with an ID attribute, give F an attribute
  of type IDREF.
• Or, let the attribute have type IDREFS, so the F
  object can refer to any number of other objects.

26
Example IDs and IDREFs

• Lets redesign our BARS DTD to include both BAR
  and BEER subelements.
• Both bars and beers will have ID attributes
  called name.
• Bars have SELLS subobjects, consisting of a
  number (the price of one beer) and an IDREF
  theBeer leading to that beer.
• Beers have attribute soldBy, which is an IDREFS
  leading to all the bars that sell it.

27
The DTD

• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR, BEER)gt
• lt!ELEMENT BAR (SELLS)gt
• lt!ATTLIST BAR name ID REQUIREDgt
• lt!ELEMENT SELLS (PCDATA)gt
• lt!ATTLIST SELLS theBeer IDREF REQUIREDgt
• lt!ELEMENT BEER EMPTYgt
• lt!ATTLIST BEER name ID REQUIREDgt
• lt!ATTLIST BEER soldBy IDREFS IMPLIEDgt
• gt

28
Example Document

• ltBARSgt
• ltBAR name JoesBargt
• ltSELLS theBeer Budgt2.50lt/SELLSgt
• ltSELLS theBeer Millergt3.00lt/SELLSgt
• lt/BARgt
• ltBEER name Bud soldBy JoesBar
• SuesBar /gt
• lt/BARSgt

29
Empty Elements

• We can do all the work of an element in its
  attributes.
• Like BEER in previous example.
• Another example SELLS elements could have
  attribute price rather than a value that is a
  price.

30
Example Empty Element

• In the DTD, declare
• lt!ELEMENT SELLS EMPTYgt
• lt!ATTLIST SELLS theBeer IDREF REQUIREDgt
• lt!ATTLIST SELLS price CDATA REQUIREDgt
• Example use
• ltSELLS theBeer Bud price 2.50/gt